1. Field of the Invention
The present invention relates to a piezo injector, and a piezo injector system including a plurality of piezo injectors and a piezo injector driving apparatus useable for a vehicle having an internal combustion engine, particularly a diesel engine.
2. Description of Related Art
As shown in FIG. 4, a commonly used piezo injector is constituted by a piezo element P, a fuel injection valve 101 configured to open and close in accordance with expansion and contraction of the piezo element P, and a housing 103 containing therein the piezo element P and the fuel injection valve 101.
To drive such a piezo injector, a piezo injector driving apparatus is used. The piezo injector driving apparatus is configured to charge and discharge the piezo element P in order to cause the piezo element P to expand and contract. Since the piezo element P is a capacitive load, if the piezo element P is supplied with a current directly from a DC power source, there is a fear that a charging current flowing into the piezo element P increases excessively, causing the piezo element P to be damaged.
Accordingly, it is necessary to prevent the charging current from exceeding an allowable range of the piezo element P at the time of charging the piezo element P. On the other hand, to increase the response of the piezo injector (fuel injection response), it is necessary to shorten the time needed for the charging current to reach a predetermined value.
For such reasons, as shown in FIG. 5, it is common that the piezo injector driving apparatus 105 includes an inductor element L having an inductance component such as a coil or a transformer disposed between the DC power source 107 and the piezo elements P in order to prevent the piezo elements P from being damaged and suppress loss of the charging current, while shorting the time needed for the charging current to reach the predetermined value. For detail, refer to Japanese Patent Application Laid-open No. 2005-16431, for example.
In a case where a current or a voltage applied to the piezo element P at the time of charging the piezo element P is increased in order to increase the opening/closing speed of the fuel injection valve 101 under a high common rail pressure (fuel regulation pressure) environment, the inductor element L has to be increased in size to prevent the piezo element P from being damaged, for the reasons described below.
The piezo injector has an ability to open and close the fuel injection valve at high speed compared to the so-called solenoid injector configured to open and close the fuel injection valve by use of a solenoid coil. Accordingly, with the piezo injector, it is possible to reduce exhaust emission. In addition, with the piezo injector, it is possible to increase an output power of an engine, because the piezo injector can inject fuel under high fuel pressure. However, for the piezo injector to operate under the condition of high fuel pressure, a voltage across the piezo element P has to be sufficiently large, because the load applied to the piezo element P is large in such a condition. And accordingly, to increase the opening/closing speed of the fuel injection valve, the charging current has to be sufficiently large so that the speed of the increase of the voltage across the piezo element P is sufficiently fast.
However, increasing the charging current requires increasing the size of the inductor element L, which causes upsizing of the piezo injector driving apparatus.